Fuel Cells in Energy Technology (9) Werner Schindler Department of Physics Nonequilibrium Chemical Physics TU München summer term 2013

Transcription

1 Fuel Cells in Energy Technology (9) Werner Schindler Department of Physics Nonequilibrium Chemical Physics TU München summer term 2013

2 - Source - Distribution - CO poisoning - Emissions (true zero, CO 2 ) Focus on - Hydrogen - Methanol (liquid, high energy density) 2

3 CO Poisoning of Pt Catalyst Remove CO from the fuel! 3

4 CO Poisoning of Pt/Ru Catalyst Remove CO from the fuel, or find CO tolerant catalysts! 4

5 Hydrogen Production Industrial production of hydrogen - from natural gas by steam reforming - from methane decomposition - from oxidation of hydrocarbons (oil) - from coal - from methanol by steam reforming - as a side product of chemical syntheses - from biomass - from water by electrolysis Water electrolysis is one of the techniques with highest energy demand. (Partial) Oxidation of hydrocarbons is exothermic. 5

6 Sources for hydrogen 6

7 Hydrogen Production Cost (No distribution / transportation cost included!) cheap expensive 7

8 Hydrogen Production Cost (different sources) 8

9 Catalytic steam reforming Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley,

10 Source and distribution available for natural gas, but reforming required to achieve a H 2 rich fuel 10

11 Some background reading on fuel reforming R. O'Hayre, W. Colella, Suk-Won Cha, F.B. Prinz: Fuel Cell Fundamentals, ch. 11, pp. 371 John Wiley & Sons, 2nd Ed., 2009 ISBN-13: J. Larmine, A. Dicks: Fuel cell systems explained, ch. 8, pp. 229 John Wiley & Sons, 2nd Ed ISBN-13: M. Kaltschmidt, H. Hartmann, H. Hofbauer: Energie aus Biomasse Springer, 2nd Ed., 2009 ISBN-13: Photographs of the book covers from 11

12 Despite - increasing energy demand (China, India, etc.) - definitely decreasing resources - and as a result definitely increasing prices 12

13 Hydrogen production from hydrocarbons - Remove impurities from the (natural) supply - Utilize efficient processes (choose appropriate fuel supply) - Maximize hydrogen output - Remove CO and CO 2 from the output 13

14 Principal chemical reactions Reactions with water / hydrogen: C n H m + n H 2 O n CO + (n+m/2) H 2 DH > 0 (steam reforming) CO + H 2 O CO 2 + H 2 DH = -38 kj / mol (water-gas shift) CO + 3 H 2 CH 4 + H 2 O DH = -206 kj / mol (methanization) Oxidations: C n H m + (n+m/4) O 2 n CO 2 + m/2 H 2 O DH << 0 (complete combustion) C n H m + n/2 O 2 n CO + m/2 H 2 DH < 0 (partial combustion) CO + ½ O 2 CO 2 DH = -286 kj / mol (CO oxidation) Carbon formation: 2 CO C + CO 2 C n H m n C + m/2 H 2 C n H m Olefines Polymers Coke (thermal cracking, pyrolysis) Depending on reaction educts and operation conditions the following processes for hydrogen production are distinguished: Steam-Reforming: C n H m + n H 2 O n CO + (n+m/2) H 2 DH > 0 (endotherm) Partial Oxidation: C n H m + n/2 O 2 n CO + m/2 H 2 DH < 0 (without steam, exotherm) C n H m + n/2 O 2 n CO + m/2 H 2 DH < 0 (with steam, autotherm) C n H m + n H 2 O n CO + (n+m/2) H 2 DH > 0 14

15 Hydrogen production via fuel reforming - overview Steam reforming (SR) Partial oxidation (POX) reforming Autothermal reforming (AR) Gasification Anaerobic digestion (AD) 15

16 1. Catalytic Steam Reforming using light hydrocarbons which vaporize completely without formation of carbon Ni based catalyst Required for protection of catalysts in the reformer (basically metal oxide sorbents like ZnO which remove H 2 S to a few ppm due to sulphurization) C n H m + n H 2 O n CO + (n + m/2) H 2 CO + H 2 O CO 2 + H 2 CO + 3 H 2 CH 4 + H 2 O n=1, m=4: methane strongly endothermic energy supplied from combustion process of gas or oil 16

17 Steam reforming of methane Equilibrium concentrations of steam reformation reactant gases as a function of temperature at 1 bar and a fixed steam to carbon ratio CH 4 + H 2 O CO + 3H 2 CO + H 2 O CO 2 + H 2 DH = 206 kj/mol DH = -41 kj/mol Catalysts: Ni or noble metals (reforming) Water gas shift at T > 400 C (HTS): Fe 3 O 4 /Cr 2 O 3 catalyst Water gas shift at T < 270 C (LTS): Cu/ZnO catalyst Source: J. Larminie, A. Dicks: Fuel cell systems explained, 2nd ed., Wiley,

18 Methanol steam reforming Methanol is a promising candidate to substitute fossil hydrocarbons Reverse methanol synthesis reaction: CH 3 OH CO + 2H 2 H = 91.7 kj / mol (endothermic > 700 C without catalyst, at C with CuNi or ZnCr alloy catalysts) Watergas shift reaction CO + H 2 O CO 2 + H 2 Steam reforming CH 3 OH + H 2 O CO 2 + 3H 2 H = kj / mol H = 50.7 kj / mol Methanol oxidation complicated pathway -> reform and use hydrogen Catalysts CuO-ZnO or CuO-Cr 2 O 3 Molar ratio of water to methanol between 0.67 and 1.5 Excess steam lowers carbon formation 1.5 m 3 H 2 per kg methanol Up to 2000 m 3 /h 18

19 A commercial fuel cell running on methanol Reformed methanol fuel cell (RMFC) Source: (May 17, 2009) 19

20 A conceptual micro-scale methanol fuel processor Source: J. Larminie, A. Dicks: Fuel cell systems explained, 2nd ed., Wiley,

21 Catalytic steam reforming in the Haldor Topsoe heat exchange reformer Haldor Topsoe is a Danish catalyst company. ca. 675 C ca. 830 C Designed for PAFC systems. Heat for the reforming reaction is provided by the combustion of lean anode exhaust gas. Source: J. Larminie, A. Dicks: Fuel cell systems explained, 2nd ed., Wiley,

22 Two examples of external reformers Honda Home Energy Station ca. 2 m 3 H 2 per hour from methane Pacific Northwest National Laboratory microfuel processor Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley,

23 2. Partial oxidation hydrocarbons The complete combustion of propane does not yield hydrogen: C 3 H O 2 3 CO H 2 O In a partial oxidation a hydrocarbon is oxidized with less than the stoichiometric amount of oxygen (incomplete combustion): Propane C 3 H 8 + 3/2 O 2 3 CO + 4 H 2 General hydrocarbon C x H y + 3/2 O 2 x CO +y/2 H 2 Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley,

24 3. Autothermal reforming is a combination of steam reforming and partial oxidation Endothermic steam reforming including water gas shift reaction: CH H 2 O(l) CO H 2 DH = kj/mol Exothermic partial oxidation: CH 4 + 1/2 O 2 CO + H 2 DH = kj/mol The stoichiometry of the sum reaction can be adjusted to give a reaction with zero reaction enthalpy: CH H 2 O(l) O 2 CO H 2 DH = 0 kj/mol (Assumption: reactants and products enter reactor at 298 K and 1 bar) For a detailed discussion see: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley, 2009, pp

25 Reforming Hotspot Fuel Processor based on methanol produces 6000 l H 2 per hour can supply1 kw fuel cell achieves 20 s after start-up 75% production General Motors Truck with gasoline reformer (arrow) 25

26 Summary on steam reforming, partial oxidation and autothermal reforming Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley,

27 A comparison of steam reforming, partial oxidation and autothermal reforming Source: R. O'Hayre et al.: Fuel cell fundamentals. 2nd ed., Wiley,

28 4. Coal gasification C + H 2 O CO + H 2 H = kj/mol Heat for this endothermic reaction is supplied by the direct combustion of a portion of the coal. Ash content, composition, agglomeration, sulphur content make it a complicated process Source: K. Kordesch, G. Simader: Fuel cells and their applications, VCH, Weinheim,

29 Gasification of biomass Source: M. Kaltschmidt: Energie aus Biomasse. 2nd ed., Springer,

30 5. Production of biogas from manure Source: R. A. Zahoransky: Energietechnik, Vieweg/Teubner,

31 Yield of biogas from digestion of biomass Digestion temperature 30 C FM: fresh mass Data from: R. A. Zahoransky: Energietechnik, Vieweg/Teubner,

32 Typical composition of biogas Data from: R. A. Zahoransky: Energietechnik, Vieweg/Teubner,

33 Hydrogen and methanol from solid biomass Most gasifiers produce hydrocarbons (primarily CH 4 ) Initial reaction step reforms these into CO and H 2 : CH 4 + H 2 O CO + 3 H 2 H = kj/mol Adjustment of CO to H 2 ratio via the water-gas shift reaction: CO + H 2 O CO 2 + H 2 H = kj/mol After this point different paths for methanol and hydrogen production Source: K. Kordesch, G. Simader: Fuel cells and their applications, VCH, Weinheim,

34 Fuel cell operation temperature may be less important when a reforming process is considered in a complete system! 34

35 SOFC advantages of internal reforming of natural gas: - Cost efficiency, minimization of components in SOFC systems; - Increase of efficiency; - Heat consumption by endothermic steam reforming process lowers the necessity for air cooling of the stack; - Faster load response. SOFC disadvantages of internal reforming of natural gas: - Carbon formation in the anode chamber; - Changes in the temperature distribution in the stack, and large temperature gradients due to the gas flow. 35